The present invention relates to a method of producing polycrystalline manganese-aluminum-carbon (Mn-Al-C) alloy magnets and, more particularly, to a method of producing Mn-Al-C alloy magnets suited for peripheral magnetization.
The Mn-Al-C alloy magnets are permanent magnets composed mainly of a face-centered tetragonal phase (.tau. phase, Llo type superstructure) which is ferromagnetic, and contain carbon as an essential component element. As magnets of this type, there are known ternary systems which do not contain any additive elements other than impurities and quaternary and higher systems containing small proportions of additive elements. The Mn-Al-C alloy magnets are available as isotropic magnets and also as anisotropic magnets having a specified direction of easy magnetization, depending on the distribution of the [001] axis of said face-centered tetragonal crystal which is the easy axis.
Generally, an anisotropic magnet has magnetic characteristics which are high only in a specified direction and exhibits its advantageous characteristics in unidirectional magnetization, i.e. in the field of bipolar magnetization. This field covers, for example, speaker magnets, motor bipolar rotor magnets and so on. However, the direction of magnetization is limited, so that in the field of peripheral multipolar magnetization, the anisotropic magnet has virtually given way to the isotropic magnet. As an anisotropic magnet used in the field of multipolar magnetization, there is known a magnet whose direction of easy magnetization extends radially in a uniform distribution from its center, but because its magnetic characteristics are low in the tangential direction, this type of magnet is not necessarily suited for multipolar magnetization applications and is rather disadvantageous particularly in high-density multipolar uses.
As methods of producing the Mn-Al-C alloy magnets, there are known a method comprising casting and heat-treatment and also a method further comprising warm plastic working process, e.g. warm extrusion. Especially the latter is known as a method of producing anisotropic permanent magnets which possess excellent characteristics such as high magnetic properties, superior mechanical strength, weather resistance and good machinability.
As a method of producing Mn-Al-C alloy magnets for peripheral magnetization other than isotropic magnets, there is known a method consisting of upsetting a cast and heat-treated alloy. It should, however, be understood that a mere upsetting does not help avoid the formation of dead zones, that it requires a comparatively high degree of working and that it might cause an uneven deformation.